1. Field of the Invention
This invention relates to methods of production and, more particular, to methods for producing a plurality of similar plastic parts by injection molding and thereafter providing different readable indicia on or within the plastic parts.
2. Prior Art
Although the invention disclosed herein is applicable to a wider range of devices, it is most exemplarly practiced in connection with device keys. Such keys are required for manual input of data and information to corresponding process systems such as, for example, typewriters, calculators, teletypewriters, desk telephone stations, electronic controls, and the like. In the production of such keys, particularly to save tool costs and to be able to quickly fulfill customers' desires, even in the case of rare special symbols or exact scripts or type styles, it is most practical to manufacture uniformly injected injection molded keys with the desired indicia being applied by subsequent operations.
Thus, it has been known in the art to print on surfaces of standard form keys subsequent to their formation. Keys produced in this manner, however, do not possess abrasive resistance such as is usually required. Thus, such keys have a shortened use life. When used in connection with high quality or high use devices, quality standards require that such keys exhibit a longer useful life and high abrasive resistance. The requirements can, at present, only be fulfilled satisfactorily by manufacturing keys with a two color injection molding process. Such manufacture, however, will of course require that a special mold or tool be prepared for each different indicia key. The tool or mold must therefore be stored for relatively long periods of time. Such special tools or molds are both expensive to initially acquire and to store and, moreover, do not represent a satisfactory method of accomodating changing style requirements.
A method for producing graphic symbols on instrument dials positioned within an otherwise closed device and identifyable through an inspection glass or a transparent cap is known, for example, from German Pat. No. 2,240,553. In this method, the surface of the dial is provided with different colored lacquer layers and the top-most layer is burned away by means of a deflectable or controllable laser beam. In this manner, the underlying layer will become visible thus providing the desired indicia. This method is, however, not practical in connection with device keys where the surface must thereafter be subjected to constant use abrasion.
Another method shown, for example, in U.S. Pat. No. 3,266,393, utilizes a laser beam to mark a film by changing the film emulsion characteristic. The shape of the marking to be applied to the film is predetermined by a slotted mask positioned within the path of the laser beam.
Additionally, U.S. Pat. No. 3,657,510 discloses a method of changing the surface of an object through vaporization, heating or chemical reaction under the influence of a laser beam.
It would be an advance in the art to provide an improved method of manufacture of similar plastic parts which are to be provided with different readable indicia by a means other than methods which require the storage of special tools for creating the different indicia parts.